Recently, there has been a noticeable increase in the application of magnetic recording/reproducing apparatus such as magnetic disk apparatus, flexible disk apparatus, and magnetic tape apparatus. The increasing importance of such apparatus is accompanied by a demand to noticeably improve the recording density of magnetic recording media used in these apparatus. In particular, since the introduction of MR heads and PRML techniques, the rise of recording density has increased markedly, and in recent years the introduction of GMR heads, TMR heads, and the like, continues to increase at a pace of approximately 100% each year.
Thus there is a demand to achieve even higher recording density than hitherto, and accordingly to achieve a magnetic recording layer which has high coercivity, a high signal-to-noise ratio (SNR), and high resolution. In hitherto longitudinal magnetic recording systems were widely used. When the linear recording density increases, demagnetization field becomes dominant and weakens the magnetization in adjacent recording magnetic domains. To avoid this, the magnetic recording layer must be made even thinner.
On the other hand, as the magnetic recording layer is made thinner, the thermal energy at room temperature and the energy barrier for maintaining the magnetic domains approach the same level. This is said to make it impossible to ignore relaxation of the recorded magnetization quantity due the effect of temperature (thermal fluctuation) and limit the linear recording density.
In view of this, an anti-ferro coupling (AFC) medium has recently been proposed as a technique for improving the linear recording density of longitudinal magnetic recording systems, and effort are being made to avoid thermomagnetic relaxation which is a problem in longitudinal magnetic recording.
Perpendicular magnetic recording techniques are attracting attention as a powerful way of improving recording density in the future. In contrast to conventional longitudinal magnetic recording systems, in which the medium is magnetized in the in-plane direction, perpendicular magnetic recording systems magnetize in a direction which is perpendicular to the surface of the medium. It is thought that this makes it possible to avoid the effects of demagnetization, which is an obstacle to achieving high linear recording density in longitudinal magnetic recording systems, and is therefore ideal for high-density recording. Since the thickness of the magnetic layer can be kept constant, the effects of thermomagnetic relaxation, which is problematic in longitudinal magnetic recording, are comparatively small.
A perpendicular magnetic recording medium generally consists of a seed layer, an intermediate layer, a magnetic recording layer, and a protective layer, which are grown successively on a nonmagnetic substrate. After these layers are grown as far as the protective layer, a lubricating layer is often applied to the surface. In many cases, a magnetic film known as a soft magnetic under layer is provided underneath the layers. The purpose of the intermediate layer is to enhance the characteristics of the magnetic recording layer. The seed layer is to control the size of the magnetic crystal as well as the crystal orientation of the intermediate layer and the magnetic recording layer.
The crystal structure of the magnetic recording layer is important in manufacturing a perpendicular magnetic recording medium having excellent characteristics. In many perpendicular magnetic recording media, the crystal structure of the magnetic recording layer has a hexagonal close-packed (hcp) structure wherein it is important that the (002) crystal plane is parallel to the substrate surface; in other words, it is important that the crystal C axis ([002]) is aligned vertically with as little deviation as possible. However, while perpendicular magnetic recording media have an advantage of allowing use of a comparatively thick magnetic recording layer, they have a drawback that the total thickness of the stacked thin-film of the entire medium tends to be thicker than that of current longitudinal magnetic recording media, and this is liable to cause deviation of the crystal structure during the medium stacking process.
One disclosed perpendicular magnetic recording medium includes an orientation control layer containing a non-crystal section, a grain size control layer, and an under layer having either a hexagonal close-packed (hcp) structure or a face-centered cubic (fcc) structure, which are provided between the soft magnetic layer and the perpendicular magnetic recording layer (e.g. see Patent Document 1).
Another disclosed perpendicular magnetic recording medium includes another crystal MgO film having a (100) plane which is broadly parallel to the substrate, this crystal MgO film being inserted between a magnetic under layer and a perpendicular magnetic recording layer (e.g. see Patent Document 2).
Thus, while various techniques have been used in the growth process in order to obtain a perpendicular magnetic recording medium having an excellent crystal structure, there is still a demand for further technological improvements to obtain even better recording and reproducing characteristics.
Patent Document 1: Japanese Unexamined Patent Publication No. 2004-30767
Patent Document 2: Japanese Unexamined Patent Publication No. 2001-23140